1. Field of the Invention
The present invention relates generally to a process for the efficient extraction and recovery of zinc, lead and cadmium products from industrial waste streams comprising zinc, lead, cadmium and iron compounds, while also providing for the enhanced product yield in an iron production process. The present invention relates more specifically to a process subjecting a waste materials stream comprising zinc compounds and iron compounds, such as electric arc furnace (EAF) dust, to a beneficiation step, for increasing the level of zinc, lead and cadmium, while decreasing the iron content in a recycling operation which recycles process solutions for reuse, and produces a briquette product from undissolved iron and carbon compounds which can be used as a supplemental feedstock for steel mills. Once the zinc, lead and cadmium compounds have been recovered, they are further purified by a process which is preferably based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This final purification process can be controlled in such a manner that the particle size and surface area of the zinc oxide produced can be controlled. Additionally, zinc compounds can be quickly, easily, and economically, synthesized from the aqueous zinc oxide slurry resulting from this process. The cadmium and lead compounds also can be recovered as chemical values.
The present invention also provides a further zinc oxide purification process which involves precipitating zinc oxide in such a manner that the desired purity and particle characteristics can be obtained. One method to control particle size is through the control of the conditions of the washing step. Additionally, although the zinc oxide purification process preferably utilizes a sodium hydroxide solution as the intermediate, the purification process of the present invention also provides for preparation of zinc oxide having particular purity and particle characteristics by utilizing intermediates such as ammonium chloride liquor, ammonium sulfate, ammonium phosphate, potassium hydroxide, ammonia/ammonium oxalate and ammonia/ammonium carbonate solutions. Once the zinc oxide has been dissolved in the solution, controlled dilution results in the precipitation of zinc oxide having predetermined purity and particle characteristics.
2. Prior Art
Zinc oxide typically is a coarse white or grayish powder which has a variety of uses including as an accelerator activator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commercial by-products including waste material streams such as fly ash and flue dust. Many of the uses of zinc oxide require that the zinc oxide have certain particular size, shape and purity characteristics. Therefore, many grades of zinc oxide having different purity and particle characteristics have been developed to meet the diverse industry requirements. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recover pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent roasting and evaporation processes were necessary. Today, most zinc oxide is made by the so called French Process which involves controlled burning of zinc metal vapor in air to obtain zinc oxide having exceptional chemical purity.
U.S. Pat. No. 3,849,121 to Burrows, now expired but which was assigned to a principal of the assignee of the present invention, discloses a method for the selective recovery of zinc oxide from industrial waste. Burrows comprises leaching a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. Burrows discloses a method to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out the iron oxides and waste metals. However, the material obtained in the last step is a mixture of a small amount of zinc oxide, hydrated zinc phases which can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of diamino zinc dichloride Zn(NH.sub.3).sub.2 Cl.sub.2 or other similar compounds containing zinc and chlorine ions. Currently, Burrows is not economically viable because of USEPA guidelines established subsequent to the issuance of Burrows. Additionally, Burrows is not a continuous method and, therefore, is not economical as a continuous process.
The first step in Burrows is the treating of the EAF dust with an ammonium chloride solution to leach the zinc oxide, lead oxide and cadmium oxide in the solution without any leaching of the iron oxides present. Twenty to fifty percent of the zinc present in the Burrows dust is in the form of an iron-zinc complex known as a spinel which is not leached by the ammonium chloride solution. Burrows therefore cannot leach and recover a significant portion of zinc present in the EAF dust.
The second step in Burrows is cementation in which the solution obtained from the initial leach is filtered to remove any remaining solids. Zinc dust then is added which causes an electrochemical reaction which causes the lead and cadmium to deposit on the zinc particles. Burrows does not teach the need to remove the lead and cadmium in this step efficiently without using a large amount of zinc. If the process requires too much zinc in this step, it will not be economically viable. The zinc powder when added tends to clump together reducing the available surface area and requiring the addition of more zinc.
The third step in Burrows takes and cools the filtrate from the cementation process and obtains what are called "zinc oxide" crystals that range in size up to 3/8 of an inch. Burrows does not produce zinc oxide of any degree of purity; x-ray diffraction figures clearly show that upon crystallization there is a mixture of many phases. Washing the crystals is not sufficient to purify the material to zinc oxide since zinc hydroxide and hydrates are also present, so that a drying step is necessary. In addition, the control of the size of the zinc oxide along with the purity is crucial. Commercial zinc oxide normally has a requirement that 99% of the particles fit through 325 mesh (44 microns). Burrows indicates no method of cooling or controlling either purity or size, and the particles produced do not meet commercial requirements. Further, a significant portion of the ammonium chloride is lost in the crystal washing step when the diamino zinc dichloride decomposes.
Waste metal process dust typically has varying amounts of lead, cadmium and other metals contained in the dust. Burrows discloses a method for removing dissolved lead and cadmium from the ammonium chloride solutions which have been used to treat the waste metal dust by the addition of powdered zinc dust to the ammonium chloride solutions. The resulting electrochemical reaction forms elemental lead deposits on the surface of the powdered zinc dust. For this reaction to proceed, a large surface area of zinc initially must be present because as the lead covers the zinc dust particle, the particle becomes no longer available for the electrochemical reaction. For this reason, very fine powder is used which immediately aggregates to form large clumps which sink to the bottom of the vessel. Rapid agitation does not prevent this from happening. Because of the aggregation of zinc, a large amount of zinc must be added to remove all of the lead, a poor practice for economic reasons. Further, if it is desired to separate the lead and some cadmium from the zinc so that all of these metals can be sold or reused., the higher the zinc concentration in the metals, the larger the mass to be processed per unit mass of zinc.
U.S. Pat. No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium.
Peters' steam distillation is directly contrary to the present temperature lowering. Steam distillation precipitates zinc carbonate, other carbonates and iron impurities, whereas temperature lowering advantageously precipitates a number of crystalline zinc compounds. Steam distillation also disadvantageously results in an increase in temperature which drives off ammonia and carbon dioxide, resulting in the precipitation of iron impurities and then zinc carbonate and other dissolved metals. The purity of the zinc carbonate obtained depends on the rate of steam distillation and the efficiency of solids separation as a function of time. Calcining at temperatures between 200.degree. C. and 1100.degree. C. converts the zinc carbonate to zinc oxide, whereas washing and drying at temperatures between 100.degree. C. and 200.degree. C. converts the zinc compounds to zinc oxide. In addition to the advantages of temperature lowering, the present process also employs a 23% NH.sub.4 Cl solution at temperatures ranging from 90-110.degree. C., and has several distinct advantages over Peters:
1. The solubility of zinc and zinc oxide is relatively high in NH.sub.4 Cl solution which is important to the efficiency of the present process in terms of the rate of the leaching, the mass of dust that can be processed, and the ability to recycle the solution. The rate of the leaching is a function of the difference between the zinc concentration in solution and the saturation concentration; the higher the saturation concentration the more rapid the leaching. The present process leaches for only 1 hour, while Peters leaches for at least several hours. In addition, the ammonium chloride solution has the added property that the solubility of zinc and zinc oxide in the solution declines rapidly with temperature, which is the basis for the crystallization-based separation which is used later in the present process.
2. Lead and lead oxide, as well as cadmium and cadmium oxide, are soluble in the ammonium chloride solution while iron oxide is virtually insoluble. During the leaching process of the present invention, 95-100% of the zinc present as zinc oxide is extracted, compared to about 55% in Peters; 50-70% of the lead present is removed, compared to less than 5% in Peters; and 50-70% of the cadmium is removed, compared to less than half in Peters. In effect, Peters does not remove a significant amount of the impurities so as to leave an acceptably clean effluent. Peters indicates that his residue, which is high in lead and is a hazardous waste, is discarded. By leaching out a significant portion of the lead and cadmium, the present process produces a material which can be used by the steel producer as they use scrap metal.
3. Peters adds powdered zinc to the solution, which has a tendency to clump reducing the surface area available for the dissolution of the zinc and the plating of the lead and cadmium. The present process teaches a method to minimize this effect through the use of an organic dispersant.
In the present process the filtrate from the cementation step is already hot (90-110.degree. C.) and contains a large amount of dissolved zinc, generally increased because of the beneficiation step, with small amounts of trace impurities. Upon controlled cooling of the solution, crystals of zinc salts begin to appear. Control of the cooling rate and temperature versus time profile is important in controlling the size distribution of the crystals and in reducing or eliminating many of the impurities which might occur. This is especially true of included solution; control of the crystallization can reduce this to virtually zero. Also, as crystallization is based on differential solubility, and none of the impurities is present in a concentration which can crystallize, the zinc salts are virtually free of any metal impurities.
The final purification step in Peters is a calcining of the zinc carbonate at 600.degree. C. to zinc oxide. In the present process, the mixture of zinc oxide hydrates and diamino zinc dichloride are suspended in hot (90-100.degree. C.) water. The zinc oxide is not soluble; however, the diamino zinc dichloride is very soluble and completely dissolves. The remaining solid, zinc oxide hydrates, is then filtered and dried at 100-350.degree. C. to remove the water of hydration. The result is a very pure zinc oxide powder of controlled particle size.
Another process offered by Engitec Impianti SpA, of Milan, Italy purports to recover zinc metal and lead cement directly from EAF flue dust using an electrowinning technology. Electrowinning is the technique of extracting a metal from its soluble salt by an electrolytic cell. Typically, it is used in the recovery of zinc by subjecting the zinc salt in solution to electrolysis and electrodepositing the elemental metal on a zinc cathode in the electrolytic cell. In the Engitec process, the EAF flue dust is leached with a spent electrolyte, such as ammonium chloride, which dissolves the zinc, lead, copper and cadmium in the EAF dust into solution while leaving the iron in solid form. The solution containing the dissolved zinc is placed in an electrolytic cell which draws the zinc from the solution onto a cathode plate, while the other heavy metals are filtered out in solid form into cement cakes. Engitec claims to obtain a zinc yield that is 99.5% pure and a lead cake consisting of a minimum of 70% lead. In effect, the Engitec process takes the product solution from the Burrows process and subjects it to electrowinning. A neutral solution of ammonium and sodium chlorides is heated to between 70-80.degree. C. The EAF dust is mixed into the chlorides solution in which the zinc and heavy metals are dissolved. The iron, calcium, magnesium and aluminum oxides are insoluble in the chlorides solution. After leaching and residue filtration, the solution is purified by cementation using zinc granules or powder. After removal of the cement, consisting of lead, copper, silver and cadmium, the purified solution is fed to the electrolysis cell.
Apparently, the electrolysis of the zinc amino complex in the purified solution occurs in a conventional open cell using a titanium permanent blank cathode and a proprietary graphite anode. In the electrolysis cell, the zinc plates on the titanium cathode. However, the deposition time for the zinc is 24 to 48 hours, depending on the current density. In addition to the electrowinning of zinc, the electrolysis cell consumes ammonia and evolves nitrogen. Because of this, in order to maintain the pH of the electrolyte in the desired range of 6 to 6.5, additional ammonium must be added to the cell in the range of 180 kg per ton of product zinc.
The use of an electrolysis cell in the Engitec process adds additional costs due to higher energy consumption, higher ammonia consumption, nitrogen handling, and maintaining the components of the electrolysis cell itself. For example, the titanium cathode and the apparently proprietary graphite anode may be costly. The Engitec process also results in the formation of metallic zinc, which has less value than zinc oxide. The residue removed from the Engitec process is composed primarily of iron oxide and zinc ferrite. Iron oxide can be used in the steel making process. The presence of zinc ferrite likely is not a significant detriment to the use of the residue from the Engitec process, but it does inject an additional impurity into any future process. It is more advantageous to obtain a residue comprising primarily iron oxide with no or an insignificant amount of zinc ferrite or other impurities.
The electrowinning of metals from chloride solutions is known in the art. U.S. Pat. No. 4,155,821 to Grontoft discloses a method for recovering chlorine using electrolytic separation. Chlorine and metal are produced from a chlorine containing electrolyte by an electrolytic process having an anode surrounded by a membrane connected to a hood. The process is maintained at a partial vacuum so that any chlorine gas generated by the anode together with some of the electrolyte is drawn away from the anode. The vacuum also is devised to control redifusion of chlorine containing electrolyte back through the membrane into the surrounding electrolyte. The process is for use with nickel recovery where the nickel chloride containing electrolyte is introduced at such a rate that the pH is maintained below a certain level. The process also may be used for cobalt recovery.
The electrodeposition of zinc from chloride solutions also is known in the art. U.S. Pat. No. 4,292,147 to Fray discloses a method for the electrodeposition of cadmium or zinc from chloride solutions derived from chlorine leaching of materials. An aqueous solution having 15 to 30% by weight of zinc or cadmium chloride is electrolyzed at a pH of 2 to 3.5 at a temperature of below 35.degree. C. with gas agitation at a current density above 100 A/m.sup.2 to form coherent zinc or cadmium at the cathode. A typical zinc containing material such as flue dust is leached with a saturated chlorine solution, preferably in the presence of chlorine hydrate. The zinc chloride solution preferably contains 20 to 30% by weight zinc or cadmium chloride and up to 20% by weight alkaline metal or ammonium chloride. The electrolysis preferably is carried out at 0.degree. C. to 9.degree. C. and above 2500 A/m.sup.2 with intermittent current reversal. Chlorine hydrate liberated at the anode may be recycled to affect leaching.
In addition to the uses of zinc oxide as a pigment, there are several important zinc compounds which are synthesized from zinc oxide. Methods to make various zinc compounds from zinc oxide are well known in the art and usually involve the reaction of zinc oxide with an acid to form a zinc compound. Generally, commercial zinc oxide is first made by the combustion of zinc vapor in air, resulting in dry powdered zinc oxide. The dry powder must first be suspended or dissolved in an aqueous solution prior to treatment with the appropriate acid to produce the desired compound. In many cases the zinc oxide is dissolved in an aqueous solution of an acid. The zinc compounds may be present in solution or precipitated as a solid.
Typical EAF flue dust usually contains less than 20% zinc-containing compounds of which less than 50% is oxidic. Typical flue dust also can contain significant amounts of iron compounds. Zinc recovered from extractive processes is much more commercially valuable if the levels of insoluble (such as iron compounds) is decreased and a higher proportion of the zinc is oxidic rather then ferric. Thus there exists a need for a method for the beneficiation of EAF dust in which levels of zinc, lead, and cadmium are increased and the iron content is decreased.
Thus, there exists a need for a method which will economically extract zinc and other products from industrial waste streams. The present method relates to the beneficiation of EAF dust to increase the levels of zinc and other products in the dust, allowing more economical recovery of the products, while decreasing the level of iron compounds in the dust. The present method also demonstrates how to increase the formation of desired zinc products and decrease the formation of undesired zinc products. Furthermore, once the desired zinc products have been obtained, zinc oxide can be further purified by a process which is based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This purification process can be controlled to produce zinc oxide having a desired size and surface area.
There also exists a need for a method which will allow the recovery of iron oxide from industrial waste streams which can be used with little or no additional treatment as the feedstock for other processes. Producing an iron oxide with a minimum amount of impurities, such as zinc ferrite, is advantageous because the iron oxide can be used as the feedstock for steel production processes. A method which results in the recovery of iron oxide would have additional value in that the iron oxide could be sold for use in other processes.
Additionally, there exists a need for an economical method of purifying zinc oxide which allows a highly purified zinc oxide to be obtained which has predeterminable and controllable purity, and particle size and shape characteristics.